This invention relates to a printer architecture, and more particularly, to a hyperacuity printer architecture.
Many of the commercially available laser printers, as well as some of the recently introduced electronic copiers, include flying spot raster output scanners (ROS's) for printing latent electrostatic images on xerographic photoreceptors. These photoreceptors generally have steeply sloped contrast vs. exposure characteristics (high gamma), together with well defined exposure thresholds (called the "xerographic threshold"), so they characteristically yield high contrast, bitmapped images (e.g., black and white). Some xerographic printers operate in a "write black" mode to optically expose the image foreground for printing by means of an "exposed area development" process, while others operate in a "write white" mode to expose the image background for printing by means of a "charged area development" process.
As is known, both write black and write white xerography are suitable for color printing. So called "full color" xerographic prints customarily are composed by printing three or four different color separations (e.g., cyan, magenta and yellow for three color printing, and cyan, magenta, yellow and black for four color printing) in superimposed registration on a suitable substrate, such as plain paper. Highlight color prints, on the other hand, can be produced by printing as few as two color separations (e.g., black and a selected highlight color). There is, however, a common thread because each of these color separations generally is a high contrast image. It, therefore, will be evident that the fundamental operating principles and functional advantages of this invention apply to both monotone and color xerography.
Many of the ROS's that have been developed for xerographic printing employ a single beam or a multi-beam laser light source for supplying one or more intensity modulated light beams, together with a scanner (such as a polygon scanner) for cyclically deflecting the modulated laser beam or beams across a photoreceptor in a fast scan direction while the photoreceptor is being advanced simultaneously in an orthogonal, process or slowscan direction. In practice, each of the laser beams typically is brought to focus on or near the photoreceptor surface to provide a substantially focused "scan spot." The scan spot or spots, in turn, scan the photoreceptor in accordance with a predetermined scan pattern because the fastscan deflection of the laser beam or beams vectorially sums with the process direction motion of the photoreceptor. Indeed, the scan pattern is dependent upon and is determined by the scan rate (scans/sec.) of the scanner, the number of scan spots that are employed, and the process speed (inches/sec.) of the photoreceptor. Such a scan pattern produces an exposure pattern because the scans are superpositioned on the photoreceptor, regardless of whether the scans simultaneously or sequentially expose the photoreceptor. Accordingly, it is to be understood that the present invention applies to printers and other displays that employ single beam or multi-beam ROS's.
Laser illuminated flying spot ROS's ordinarily are designed to provide generally circular or elliptical scan spots. To a first approximation, such a scan spot is characterized by having a gaussian intensity profile (as is known, this may be a very rough approximation if the scan spot is truncated). Prior laser printers generally have employed scan patterns that are selected to have a scan pitch (i.e., the center-to-center displacement, in the process direction, between spatially adjacent scan lines) that is comparable to the diameter of the scan spot as determined at an intensity level that is equal to one-half of its maximum or peak intensity. This sometimes is referred to as the full width, half max. ("FWHM") diameter of the scan spot.
Images contain many transitions. For instance, black and white and other dual tone images have transitions at the boundaries between their foreground features and their backgrounds, such as the transitions that demark line edges, font contours, and halftone dot patterns. Color images commonly include still additional transitions at the boundaries between differently colored foreground features. Consequently, the perceived quality of monotone and color prints tends to be strongly dependent upon the precision with which the printing process spatially positions these transitions.
Modern laser xerographic printers typically are designed to print at spatial resolutions ranging from about 300 dots/inch ("d.p.i") to about 600 d.p.i. As a practical matter, the image transition positioning precision of these printers can be increased to an extent by increasing their spatial resolution, but the frequency responses of the photoreceptor/developer combinations that currently are available for xerographic printing usually impose an upper limit on the resolution that can be achieved. Moreover, even when increased resolution is technically feasible, the additional resolution imposes further and potentially burdensome requirements on the optical and electrical design requirements of these printers, so there usually is a cost/performance tradeoff to be considered. Specifically, the cost of xerographic print engines tends to escalate as their spatial resolution is increased because of the additional memory and bandwidth these printers require for faithfully rendering higher resolution bitmap images without sacrificing throughput.
To give a historical overview, five stages in the evolution of xerographic laser printing over the last twenty years are shown in FIG. 40. The first two stages show both low resolution desktop printers of stage 1 and high resolution offset printers of stage 2 utilizing single bit per pixel image data. These printers print with data resolutions which have the same pitch as the imager spot. The offset printers of stage 2 are expensive and require long print cycles due to their high resolution of 800 to 4000 bits per inch. Because of low cost requirements, some of the desktop printers of stage 1 are limited to coarse binary data. This low resolution data, at one bit per pixel and 300 bits per inch (bpi), generally lacks the fidelity required to make acceptable images. Therefore, both the stage 1 and stage 2 printing systems could be improved upon.
According to human vision research, the frequency response or resolution of a printing system need only exceed just beyond the resolving power of the human visual system. Such resolving power is known as visual acuity. However, there are human visual considerations that require the placement of edges 10 to 60 times more accurately than that indicated by frequency response considerations. These requirements are based on hyperacuity, or the visual systems' ability to differentiate locally misaligned edges to a much greater extent than the interreceptor spacing of the eye. In this case, it is not the frequency response (resolution) of the visual system that is most important, but the ability to reckon edges with high precision. Therefore, there was a need to be able to place edges or transitions in images in both the fastscan and process directions with precision greater than that of the actual printer resolution. A detailed description of the relationship between the human visual system and printer resolution can be found in the Journal of Electronic Imaging, April, 1993, Vol. 2(2), pages 138-146, in an article titled "Hyperacuity Laser Imager," by Douglas N. Curry, which is hereby incorporated by reference.
In the late 1980's, printers were marketed which could move image edges in the process direction with subscan precision by inserting a gray modulator between the image source and the laser diode, as shown in the thirdstage. This provided a new dimension for improving the quality of the images, however, the input data, at 300 bpi, did not have enough fidelity to drive it. Therefore, a second innovation as depicted by stage 3, template matching, was provided which restores some of the fidelity to the 300 bpi data which was thrown away or never inserted when it was originally scan converted. Such systems are shown in U.S. Pat. No. 4,847,641 to Tung, U.S. Pat. No. 4,437,122 to Walsh et al., and U.S. Pat. No. 4,450,483 to Coviello. Template matching generally passes a window over the data, looking for matches between an internally stored data pattern and the imaging data. When a match is found, the central pixel of data in the window is modified and output to the printer. However, template matching does not improve halftones, sometimes makes mistakes and is generally limited to removing jaggies in stairstepped edges that are nearly horizontal or vertical. In addition, it requires "tuning" of the target printer by calibrating the code stored in the template matching circuitry, a process which is costly and tends to lock the manufacturer out of making improvements to the development process over the product lifetime.
The fourth stage shows how the backwards compatibility enabled by template matching was rendered unnecessary by providing source data which has a higher resolution (for instance 600 to 1200 bits per inch) than the imager spot which remained at a low resolution spot size. This allows the production of high fidelity images from data which has been scan converted at very high resolution (600-1200 bpi), well beyond the acuity needs of human vision. The data are placed into a large, super-resolution data buffer, then read out several rasters at a time through a filter. With this method, excessive resolution is used to position edges, therefore the memory requirements go up as the square of the desired edge precision. Since edge placement precision on the order of 2000 to 4000 per inch are required to enter the Offset Printing market (or to make good halftones in any market), this method tends to be wasteful and slow. In addition, the already halftoned, bitmapped data is generally produced for a particular printer and is therefore device dependent.